In commercial urea processes, urea (NH2CONH2) is produced by reacting ammonia (NH3) and carbon dioxide (CO2) at elevated temperature and pressure according to the reactions:2NH3+CO2→NH2COONH4 NH2COONH4→NH2CONH2+H2O
An overview of commercial processes using this chemistry is given e.g. in Ullmann Encyclopedia, 2005 Wiley-VCH verlag, Weinheim, Germany, chapter Urea. All processes use NH3 and CO2 as feedstock. These feedstocks, usually both originating from a ammonia plant contain impurities. The impurities do not react in the abovementioned chemistry, and therefore need to be purged from the urea plant in order to prevent their accumulation.
The invention relates to a urea plant comprising synthesis equipment, synthesis piping, a CO2 and a NH3 feed, and a purge stream. In this purge stream, part of, or all inert gases that are present in the CO2 and NH3 feed, as well as part, or all, of any other inert gases fed to the urea plant are released from the urea plant. Inert gases, in this context are defined as any gaseous components that do not contribute to the chemical production of urea.
Both feedstocks are usually originating from an ammonia plant. Although an ammonia plant is a net heat producer and a urea plant is a net heat (steam) consumer, and it is normal practice to integrate the steam systems of both plants, net heat is required, which is generally obtained from burning fuel.
In order to reduce the production of greenhouse gases caused by burning fuel, it is a purpose of the invention to reduce the fuel consumption required for the production of urea.
This purpose is obtained by connecting the purge line with a fuel gas feed of a utility plant or an ammonia plant, the fuel consumption can be reduced by 2%, which for an average urea/ammonia complex corresponds with a saving of about 7×106 kg natural gas/year.
The NH3 feed typically comprises also minor amounts of CH4.
Methane (CH4) contributes to the growing global background concentration of tropospheric ozone (O3), an air pollutant associated with premature mortality. Methane and ozone are also important greenhouse gases.
A further advantage of the invention is that methane emissions of the urea plant are reduced, which decreases surface ozone and slowing global climate warming.
The CO2 feed is generally provided with an additional oxygen stream, generally originating from air. The oxygen serves as an agent to prevent excessive corrosion of the synthesis equipment and the synthesis piping. As the oxygen does not contribute to the production of urea, it is vented with the purge gas. Excessive corrosion is prevented when the oxygen concentration in the purge gas is in the range of 5-20 mol %
As the NH3 feed and the CO2 feed comprise minor amounts of H2, the purge stream is very likely to be inflammable even before the addition of an oxidizing agent (e.g. air). Because of this inflammable character of the purge gas stream, it is unsafe to transport this gas (e.g. via pipelines) over some distance. In contrast, it is common practice up to now to vent this purge gas via shortest possible connections into the atmosphere.
By the use of a duplex ferritic-austenitic steel with a high content of Cr and N and a low content of Ni, as described in WO9500674, as a material of construction for the synthesis equipment and synthesis piping, oxygen needs no longer be supplied to the synthesis to prevent corrosion, or only in very low concentrations in the carbon dioxide feed e.g. <0.05 vol % of oxygen. Said duplex ferritic-austenitic steel is preferably a duplex, stainless steel alloy that contains, in % by weight: 0-0.05 C; 0-0.8 Si; 0.3-4 Mn; 28-35 Cr; 3-10 Ni; 1.0-4.0 Mo; 0.2-0.6 N; 0-1.0 Cu; 0-2.0 W; 0-0.010 S; 0-0.2 Ce, the remainder being Fe and normally occurring impurities and additives, the ferrite content being 30-70% by volume.
The use of said duplex ferritic-austenitic steel allows a reduction of the additional oxygen stream in the CO2 feed such that the oxygen concentration in the purge gas can be reduced to between 0-10 mol % or 0-1 mol %, without the risk of excessive corrosion taking place. Preferably oxygen is essentially absent in the purge gas. It is surprising that the purge gas stream, essentially without oxygen, now can be transported (e.g. by pipelines) without associated safety risks, to be used as a fuel gas.
The purge line of the urea plant of the invention is connected with a fuel gas input line of a utility plant or an NH3 plant. Preferably the purge stream is directed to the reformer section in an ammonia plant, or to the fuel gas supply of a steam boiler.
The invention will be elucidated hereinafter on the basis of FIG. 1, without being restricted to this embodiment.
FIG. 1 shows a urea plant (1) comprising a CO2 feed (10) and a NH3 feed (11). Solid urea leaves the plant via line 12. Water is purged via line 13. Purge line (14) is connected with a fuel gas input line of a utility plant or an NH3 plant (2).